I remember watching a young officer try to lift a heavy shield with his left hand while his right held the reins, trembling and failing until he forced himself to practice in secret until the muscle obeyed without thought. It is the same with duty when it seems impossible; we despise the task because we do not yet know our own strength, yet the very act of trying rewires us for the hour that demands us.
Do not wait for confidence before you begin what your nature requires. The left hand holds the bridle not because it was always strong, but because it was used until despair turned into habit. Go and do what you fear is beyond you, and let the doing make you capable of holding the reins when the road grows steep.